If you are not sure whether you have found a bug, here are some guidelines:

If the compiler gets a fatal signal, for any input whatever, that is a
compiler bug. Reliable compilers never crash.

If the compiler produces invalid assembly code, for any input whatever
(except an asm statement), that is a compiler bug, unless the
compiler reports errors (not just warnings) which would ordinarily
prevent the assembler from being run.

If the compiler produces valid assembly code that does not correctly
execute the input source code, that is a compiler bug.
However, you must double-check to make sure, because you may have run
into an incompatibility between GNU C and traditional C
(see section Incompatibilities of GNU CC). These incompatibilities might be considered
bugs, but they are inescapable consequences of valuable features.
Or you may have a program whose behavior is undefined, which happened
by chance to give the desired results with another C or C++ compiler.
For example, in many nonoptimizing compilers, you can write `x;'
at the end of a function instead of `return x;', with the same
results. But the value of the function is undefined if return
is omitted; it is not a bug when GNU CC produces different results.
Problems often result from expressions with two increment operators,
as in f (*p++, *p++). Your previous compiler might have
interpreted that expression the way you intended; GNU CC might
interpret it another way. Neither compiler is wrong. The bug is
in your code.
After you have localized the error to a single source line, it should
be easy to check for these things. If your program is correct and
well defined, you have found a compiler bug.

If the compiler produces an error message for valid input, that is a
compiler bug.

If the compiler does not produce an error message for invalid input,
that is a compiler bug. However, you should note that your idea of
"invalid input" might be my idea of "an extension" or "support
for traditional practice".

If you are an experienced user of C or C++ compilers, your suggestions
for improvement of GNU CC or GNU C++ are welcome in any case.